Integrated circuit designers are continually shrinking the size of integrated circuit devices to improve speed and increase device density on a chip. As more devices are formed in a smaller area, there is less space available above the devices for the electrically conductive interconnect leads that connect the devices; therefore, thinner interconnect leads are required. However, thinner interconnect leads, especially in relatively long runs, are more prone to failure stemming from stress induced by thermal cycling during subsequent wafer processing.
Thermal cycling induces stress because of the differences between the thermal coefficient of expansion ("TCE") of the conductive lead material and that of other materials on the wafer. For example, aluminum, which is typically used as interconnect lead material, has a TCE of 230.times.10.sup.-7 .degree. C.sup.-1 and is typically deposited onto a dielectric layer of silicon dioxide, which has a TCE of only 5.times.10.sup.-7 .degree. C.sup.-1. Stress is created in the aluminum lead and the dielectric because each tends expand and contract at different rates as temperature is changed.
Thermally induced stress in thin interconnect leads can cause them to crack. Cracks either sever the electrical connection completely or reduce the current-carrying cross-sectional area of the lead, thereby producing high current densities that cause electro-migration and eventual lead failure. Thermal cycling also promotes the formation of hillocks, i.e., small mounds, in thin leads. Because hillocks protrude upward, they interfere with the deposition of subsequent layers, causing pinholes and voids. Pinholes and voids are susceptible to corrosion and can cause high local current densities that cause electro-migration and eventual failure. Hillocks can also penetrate through insulating layers and cause short circuits.
It has previously been considered that hillocks occur only when the crystal grain size of the conductor is less than the conductor width, and, therefore, that hillocks would occur less frequently as leads are made thinner. However, Pico and Bonifield, in "Thermal Hillocks on Half-Micron Aluminum Lines: The Next Reliability Issue?" Proceedings of the Institute of Electronic and Electrical Engineers VMIC Conference, Jun. 11-12, 1991, pp. 256-57, have demonstrated that hillocks form on leads that are smaller than the lead grain size. They reported that hillocks ranging in size from 0.5 .mu.m to 1.3 .mu.m are thermally induced in aluminum leads less than 0.6 .mu.m wide and occur as frequently as once every 90 .mu.m to 300 .mu.m along the lead. Thus, it now appears that hillock formation will be a major problem for the thinner leads required as integrated circuits become denser.
Interconnect leads for integrated circuits are typically made by physical vapor deposition of an aluminum alloy at a uniform thickness onto an insulating layer. Hillock formation has been reduced by adding a small amount of copper to the aluminum conductor, but copper increases the resistivity of the aluminum lead, changes the etch back time, and requires a more complex deposition process.